我正在阅读有关AsyncTask的文章,并尝试了下面的简单程序。但这似乎并不奏效。我该怎么做呢?

public class AsyncTaskActivity extends Activity {

    Button btn;

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        btn = (Button) findViewById(R.id.button1);
        btn.setOnClickListener((OnClickListener) this);
    }

    public void onClick(View view){
        new LongOperation().execute("");
    }

    private class LongOperation extends AsyncTask<String, Void, String> {
        @Override
        protected String doInBackground(String... params) {
            for(int i=0;i<5;i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    // TODO Auto-generated catch block
                    e.printStackTrace();
                }
            }
            TextView txt = (TextView) findViewById(R.id.output);
            txt.setText("Executed");
            return null;
        }

        @Override
        protected void onPostExecute(String result) {
        }

        @Override
        protected void onPreExecute() {
        }

        @Override
        protected void onProgressUpdate(Void... values) {
        }
    }
}

我只是试图在后台进程中5秒后更改标签。

这是我的main.xml:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:layout_width="fill_parent"
              android:layout_height="fill_parent"
              android:orientation="vertical" >
    <ProgressBar
        android:id="@+id/progressBar"
        style="?android:attr/progressBarStyleHorizontal"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:indeterminate="false"
        android:max="10"
        android:padding="10dip">
    </ProgressBar>
    <Button
        android:id="@+id/button1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Start Progress" >
    </Button>
    <TextView android:id="@+id/output"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Replace"/>
</LinearLayout>

当前回答

AsyncTask:

public class MainActivity extends AppCompatActivity {

 private String ApiUrl="your_api";

   @Override
   protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.activity_main);
     MyTask myTask=new MyTask();
     try {
         String result=myTask.execute(ApiUrl).get();
         Toast.makeText(getApplicationContext(),result,Toast.LENGTH_SHORT).show();
     } catch (ExecutionException e) {
         e.printStackTrace();
     } catch (InterruptedException e) {
        e.printStackTrace();
     }

  }


   public class MyTask extends AsyncTask<String,Void,String>{


    @Override
    protected String doInBackground(String... strings) {
        String result="";
        HttpURLConnection httpURLConnection=null;
        URL url;

        try {
            url=new URL(strings[0]);
            httpURLConnection=(HttpURLConnection) url.openConnection();
            InputStream inputStream=httpURLConnection.getInputStream();
            InputStreamReader reader=new InputStreamReader(inputStream);
            result=getData(reader);
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return result;
    }

    public String getData(InputStreamReader reader) throws IOException{
        String result="";
        int data=reader.read();
        while (data!=-1){
            char now=(char) data;
            result+=data;
            data=reader.read();
        }
        return result;
    }
  }
}

其他回答

在使用AsyncTask时,有必要创建一个类继任者,并在其中注册我们所需的方法的实现。本节课我们将学习三种方法:

doInBackground -将在一个新线程中执行,在这里我们解决了所有困难的任务。因为非主线程不能访问UI。

onPreExecute -在doInBackground之前执行,并可以访问UI

onPostExecute -在doInBackground之后执行(如果AsyncTask被取消就不工作-关于这个在下一课中),并可以访问UI。

这是MyAsyncTask类:

class MyAsyncTask extends AsyncTask<Void, Void, Void> {

  @Override
  protected void onPreExecute() {
    super.onPreExecute();
    tvInfo.setText("Start");
  }

  @Override
  protected Void doInBackground(Void... params) {
    // Your background method
    return null;
  }

  @Override
  protected void onPostExecute(Void result) {
    super.onPostExecute(result);
    tvInfo.setText("Finish");
  }
}

这是如何调用你的Activity或Fragment:

MyAsyncTask myAsyncTask = new MyAsyncTask();
myAsyncTask.execute();

移动这两条线:

TextView txt = (TextView) findViewById(R.id.output);
txt.setText("Executed");

你的AsyncTask的doInBackground方法,并把它们放在onPostExecute方法。你的AsyncTask应该看起来像这样:

private class LongOperation extends AsyncTask<String, Void, String> {

    @Override
    protected String doInBackground(String... params) {
        try {
            Thread.sleep(5000); // no need for a loop
        } catch (InterruptedException e) {
            Log.e("LongOperation", "Interrupted", e);
            return "Interrupted";
        }
        return "Executed";
    }      

    @Override
    protected void onPostExecute(String result) {               
        TextView txt = (TextView) findViewById(R.id.output);
        txt.setText(result);
    }
}

简单:

LongOperation MyTask = new LongOperation();
MyTask.execute();
private class AsyncTaskDemo extends AsyncTask<Void, Void, Void> {

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        // Showing progress dialog
        progressDialog = new ProgressDialog(this);
        progressDialog.setMessage("Loading...");
        progressDialog.setCancelable(false);
        progressDialog.show();
    }

    @Override
    protected Void doInBackground(Void... arg0) {

        // Do code here

        return null;
    }

    @Override
    protected void onPostExecute(Void result) {
        super.onPostExecute(result);
        // Dismiss the progress dialog
        if (progressDialog.isShowing()) {
            progressDialog.dismiss();
        }
    }

    @Override
    protected void onCancelled() {

        super.onCancelled();
        progressDialog.dismiss();
        Toast toast = Toast.makeText(
                          getActivity(),
                          "An error is occurred due to some problem",
                          Toast.LENGTH_LONG);
        toast.setGravity(Gravity.TOP, 25, 400);
        toast.show();
    }

}

AsyncTask:

public class MainActivity extends AppCompatActivity {

 private String ApiUrl="your_api";

   @Override
   protected void onCreate(Bundle savedInstanceState) {
     super.onCreate(savedInstanceState);
     setContentView(R.layout.activity_main);
     MyTask myTask=new MyTask();
     try {
         String result=myTask.execute(ApiUrl).get();
         Toast.makeText(getApplicationContext(),result,Toast.LENGTH_SHORT).show();
     } catch (ExecutionException e) {
         e.printStackTrace();
     } catch (InterruptedException e) {
        e.printStackTrace();
     }

  }


   public class MyTask extends AsyncTask<String,Void,String>{


    @Override
    protected String doInBackground(String... strings) {
        String result="";
        HttpURLConnection httpURLConnection=null;
        URL url;

        try {
            url=new URL(strings[0]);
            httpURLConnection=(HttpURLConnection) url.openConnection();
            InputStream inputStream=httpURLConnection.getInputStream();
            InputStreamReader reader=new InputStreamReader(inputStream);
            result=getData(reader);
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return result;
    }

    public String getData(InputStreamReader reader) throws IOException{
        String result="";
        int data=reader.read();
        while (data!=-1){
            char now=(char) data;
            result+=data;
            data=reader.read();
        }
        return result;
    }
  }
}